1. Field of the Invention
The present invention relates to a process for deinterlacing the frees of a moving image sequence.
It applies in particular to the conversion of television image formats.
2. Discussion of Background
The format usually used in television is termed "interlaced" scanning, that is to say that alternately a "frame" consisting only of even-numbered lines or of odd-numbered lines of the image is transmitted instead of the complete image. This halving in the quantity of information to be transmitted takes place however to the detriment of the quality of the images, and this is all the more noticeable when moving images are involved. The invention entails a process for converting the "interlaced" scanning format into the so-called "progressive" or "sequential" natural format in which all of the lines are present with each image. The intended objective is to reconstruct, from a sequence of interlaced source images, a sequence of progressive images with a visual quality which is as close as possible to what may be an ideal sequence of progressive source images, which was never interlaced.
The existing techniques of reconstruction may be classified into four categories depending on whether they employ linear, motion-adapted, non-linear motion non-compensated or motion-compensated filterings. Amongst the techniques of linear filtering, inter-frame pure temporal filtering is ideal for sequences with very weak motion, but highly visible artefacts quickly occur once the scene moves, essentially on the contours of the objects in motion which tend to become double (object ghosts). Intra-frame (or vertical) pure linear filtering never produces such troublesome artefacts and is therefore chosen in preference for sequences with substantial motion but, however, the resolution of the image thus deinterlaced remains limited in principle and certain spectral aliasing effects (staircase phenomenon on the diagonal contours) cannot be suppressed. Therefore, a spatio-temporal linear compromise can only accumulate the defects in the two types of filtering. Motion-adapted filtering consists in toggling between various modes of linear filtering within the same image depending on the motion, opting for pure temporal filtering for weak motions, pure spatial filtering for strong motions and spatio-temporal filtering for intermediate motions. Independently of the problems of visibility of switching between the various modes, the intrinsic problems of linear filtering in respect of sequences in motion are not resolved.
Numerous non-linear filtering techniques have been proposed, among them the best known are founded on the principle of median filtering, to improve the deinterlacing without estimating the motion in the scene represented. However, the results obtained are still fairly mediocre. It is commonly accepted that only motion-compensated techniques, that is to say which fetch missing information from the adjacent images, which precede and follow in the direction of motion of the objects of the scene, allow conversions of spatio-temporal formats without degradation. It appears in fact that only an accurate knowledge of the motion can make it possible to adapt the conversions to the spatio-temporal spectrum of the moving images. Several approaches along these lines, founded on motion compensation, are known, essentially in the publications produced in the context of the European RACE II "Transit" project. However, the major problem which has not yet been resolved is that of the control of a fallback mode, made necessary by the inevitable errors in estimating the inter-image motion.
The aim of the invention is to alleviate the aforesaid drawbacks.